Petroleum hydrocarbon feedstocks boiling in the range above about 400.degree.F. have been converted to lower hydrocarbons by heating them at a temperature of 600.degree.F. to about 1100.degree.F. in contact with amorphous silica-alumina cracking catalyst. The method of preparing the catalyst may be the determining factor in whether a particular composite may be useful commercially for a given process.
The prior art teaches various methods for the preparation of silica-alumina hydrogels. Generally, the most widely used method is similar to that disclosed in U.S. Pat. No. 3,650,989 and consists of first preparing a silica hydrogel by acidification of the alkali metal silicate which is then followed by a slurring of the gel in a solution of an aluminum salt and which is in turn followed by the addition of an ammoniacal base in excess to neutralize the salt and precipitate the oxide. Alternatively the aluminum salt (usually Al.sub.2 (SO.sub.4).sub.3) is directly added to the alkali metal silicate followed by the same precipitation procedure described above. All of these prior art methods must resort to time consuming steps to insure Na.sub.2 O removal. In addition, the catalysts formed by these procedures are not very hydrothermally stable.
The presence of small amounts of impurities may result in a significantly detrimental effect on the catalytic activity and this is particularly the case when these impurities are composed of the alkali metals. The presence of alkali metal ions results in a lowering of the acidity of the silica-alumina catalyst which consequently results in a lowering of the catalytic activity.
Heretofore, silica-alumina formed by known methods have been treated so that the sodium content (from the sodium silicate) is reduced to less than 1% and generally to about 0.05% otherwise such catalysts are not commercially acceptable. In order to achieve such low sodium concentrations prior methods have had to resort to numerous washing steps in order to remove the ion exchanged Na.sub.2 O.